Machines such as, for example, wheel loaders, dozers, backhoes, dump trucks, and other heavy equipment often utilize a hydraulic system having one or more hydraulic cylinders to assist the performance of various tasks. Such hydraulic systems typically use load sensing pumps and controls to affect the flow of fluid being directed to the hydraulic cylinders. The fluid pressure generated by a load sensing pump is often dictated by the hydraulic cylinder having the largest load. However, the other hydraulic cylinders may not require such a magnitude of fluid pressure. Therefore, the pressure of fluid being directed to the other hydraulic cylinders may need to be reduced.
One method used to reduce the fluid pressure is the employment of flow control valves, which throttle the flow of the fluid. Throttling the fluid removes energy from the flow and ultimately reduces the fluid pressure. However, once the energy from the flow is removed, it is not recovered and becomes wasted energy. The efficiency of the system can be improved if the energy removed during throttling is recovered for later use.
Another issue facing hydraulic systems utilizing multiple hydraulic cylinders involves providing a temporary pressure boost to the hydraulic cylinders. Some applications may require a temporary boost of force generated by a particular cylinder. However, relying on the load sensing pump to provide such a pressure boost may be inefficient. In particular, because the load sensing pump serves all hydraulic cylinders of the hydraulic system, any pressure increase generated by the load sensing pump will increase the pressure of fluid being supplied to cylinders not needing a pressure boost. In addition, at least a portion of the energy used to boost the fluid pressure is lost when the fluid being supplied to hydraulic cylinders not needing the pressure boost is throttled by flow control valves. Furthermore, sizing the load sensing pump to generate the necessary pressure in the system for such a boost may be inefficient because such a pressure would be needed only for a small percentage of the pump's duty cycle.
One method for recovering energy from a hydraulic circuit can be found in U.S. Pat. No. 6,460,332 (the '332 patent) issued to Maruta et al. on Oct. 8, 2002. The '332 patent discloses a pressure energy recovery apparatus within a hydraulic circuit. The pressure energy recovery apparatus includes a hydraulic motor/pump operationally connected to a motor/generator. The motor/generator is operationally connected to a battery. In a first mode, the motor/pump is actuated by an inflow of fluid flowing out of a hydraulic actuator. The motor/pump drives the motor/generator to produce energy, which is stored within the battery. In a second mode, stored energy within the battery actuates the motor/generator, which drives the motor/pump. The motor/pump produces a outflow of pressurized fluid to supplement the flow of pressurized fluid supplied by the hydraulic circuit's main pump.
Although the pressure energy recovery apparatus of the '332 patent may recover energy from fluid exiting from the hydraulic actuator, the energy recovery capacity of the hydraulic circuit may be limited. In particular, the pressure energy recovery apparatus does not control the pressure of fluid entering the hydraulic cylinder. Instead, conventional flow control valves are used, which do not recover the energy removed from the fluid during throttling. Therefore, the energy recovery capacity and ultimately the efficiency of the system is limited. Furthermore, using a separate flow control valve may increase costs and complexity of the system.
In addition, the system of the '332 patent directs the recovered energy to the main drive pump. As discussed above, at least a portion of the recovered energy is wasted when a particular cylinder requires a boost in pressure. This is because the recovered energy is also used to boost the pressure of fluid flowing to other hydraulic cylinders. The recovered energy is lost when flow control valves associated with the other hydraulic cylinders throttle the fluid flows, thereby removing the recovered energy contained within such flows.
The disclosed system is directed to overcoming one or more of the shortcomings set forth above and/or other shortcomings in the art.